"And he shall confess that he has sinned"—Leviticus 5:5.
One who has sinned is obligated to verbally confess and say, "Please, G‑d, I have sinned by doing..." The individual should then elaborate to the best of his ability and ask G‑d for atonement. This mitzvah applies whether the sin was against G‑d or against a fellow man.
Even in the times of the Holy Temple when a person would bring a sin-offering to attain forgiveness, he was still required to orally confess his sin. The same for those who were guilty of capital offenses, they were asked to confess their sin prior to their execution.
